Purpose of review
Thyroid hormone receptors mediate a wide variety of biologic processes in cells. They are members of the nuclear hormone receptor gene family, which includes the receptors that mediate the effects of steroid hormones, vitamin D, and the retinoids. These receptors are DNA binding proteins, which act as ligand-dependent transcription factors. One of the retinoid receptors, retinoid X receptor, acts as a dimerization partner for the thyroid hormone receptors and several other members of the nuclear receptor family. This review gives a historical perspective and overview of the thyroid hormone receptors and reviews the role of the retinoid X receptor in influencing the action of the thyroid hormones.
Retinoid X receptor forms heterodimers with the thyroid hormone receptors and several other nuclear receptors and enhances the binding of these receptors to DNA. In addition, for thyroid hormone receptor/retinoid X receptor heterodimers, the general consensus has been that activation occurs through the thyroid hormone receptor component of the heterodimer and that retinoid X receptor does not bind its ligand or mediate cross-talk with the thyroid hormone receptor component of the heterodimer. Several recent studies, however, indicate that retinoid X receptor can bind its cognate ligand and modulate the activity of the thyroid hormone receptor/retinoid X receptor heterodimer through a variety of novel mechanisms.
The retinoid X receptors play an important role in mediating the action of the thyroid hormones and in part mediate these effects through cross-talk with thyroid hormone receptor in the heterodimer. Future studies that expand on recent findings in the field should help provide new insights into transcriptional regulation by thyroid hormone nuclear receptors.